We have studied the structural and ligand-binding dynamics of two model systems for human hemoglobin, component I of trout hemoglobin and a hybrid hemoglobin with cobalt substituted for iron in the alpha subunits, by measuring time-resolved optical absorption spectra following photodissociation of carbonmonoxide as a function of degree of photodissociation. For trout I hemoglobin, a global description of the temporal evolution of the spectrum of the photolyzed molecule for degrees of photodissociation ranging from 10% to 100% requires seven exponential relaxations involving ligand rebinding and/or protein conformational changes. The first two relaxations involve both geminate rebinding of dissociated ligands and tertiary structural changes in photolyzed subunits of the protein. The third relaxation has been shown to reflect the quaternary conformational change of both zero- and singly-liganded molecules from the R to the T structure. We have derived the activation energy of the R-T change in the zero-liganded molecule from the temperature dependence of the rate of this relaxation, and we have determined that the transition state in this process is energetically (and therefore probably structurally) much more similar to the R state than the T state. The cobalt-substituted hybrid hemoglobin hasonly two binding sites for carbon monoxide, and from the spectra measured atpartial photolysis we have determined separately for the zero and singly-liganded molecules the time courses of both ligand rebinding and protein conformational changes. We have also derived oxygen binding curves for single crystals of human hemoglobin from polarized optical absorption spectra of crystals at various oxygen pressures. We have observed that hemoglobin in the crystal binds oxygen essentially non-cooperatively and with very low affinity.